A large number of techniques for fastening a nut to a bolt and fastening the nut not to loosen have been proposed. The techniques are carried out and well known. Among the techniques, a fastening mechanism for fastening two nuts to the same bolt one on top of the other and preventing the nuts from loosening by use of a frictional force of the nuts is based on a method generally and frequently practiced. The tightening of the nuts is generally performed by manual operation using a wrench. However, in fastening operation in mass production, nuts are fastened using a power-rotated fastening tool. This kind of fastening work is usually performed for each nut one at a time.
As an example of the fastening tool, an impact wrench that applies an intermittent striking force to a fastening member is known. It is well known that a fastening action of the impact wrench is performed using a driving force of an electric motor, an air motor or the like (e.g., Japanese Patent Application Laid-Open No. 2006-88279: Patent Document 1). The fastening action is widely applied in automobile-related fields. A method of placing two nuts one on top of the other and fastening one of the nuts as an unremovable lock nut is also known (e.g., Japanese Patent Application Laid-Open No. 2000-240632: Patent Document 2). Further, as another fastening form, a method of fastening a member on a nut side to a structure making use of relative movements of a bolt and a screw of the nut is also known (e.g., Japanese Patent Application Laid-Open No. 2003-156024: Patent Document 3). Furthermore, a double nut tightening tool for forming two threads having different pitches in a bolt, simultaneously screwing two nuts having different pitches into the threads, and tightening the nuts is also proposed (Japanese Patent Application Laid-Open No. 2006-167894: Patent Document 4).
As explained above, a conventional fastening tool was mainly a tool for separately turning and fastening a nut or a bolt. Tools of various forms have been proposed according to modes of fastening. However, these tools were applied basically to a mode of fastening a single part. Thus, for example, even in a locking structure by double nuts, a structure for separately fastening the individual nuts is adopted for two nuts.
As one of such a locking and fastening structure, a structure employing a fastening method for preventing loosening with double nuts is proposed and carried out in which two nuts of a coarse thread and a fine thread formed in different pitches were screwed onto a bolt which has two types of threads having different pitches formed in one bolt, for example, a bolt provided with a coarse thread and a fine thread. A double nut fastening tool for the structure is also proposed (Patent Document 4). The proposed structure is not practical because adjustment of tightening torque sometime cannot be separately performed for each of the nuts.